Estimation of temporally averaged sediment delivery ratio using aggradational terraces in headwater catchments of the Waipaoa River, North Island, New Zealand

The sediment delivery ratio was estimated for two periods (28 years and eig ht years) following reforestation of seven tributary catchments (0.33 to 0. 49 km(2)) in the headwaters of the Waipaoa River basin, North Island, New Z ealand. In these catchments, gully erosion, which largely resulted from cle arance of the natural forest between 1880 and 1920, is the main source of s ediment to streams. Reforestation commenced in the early 1960s in an attemp t to stabilize hillslopes and reduce sediment supply. Efforts have been par tially successful and channels are now degrading, though gully erosion cont inues to supply sediment at accelerated rates in parts of the catchment. Data from the area indicate that the sediment delivery ratio (SDR) can be e stimated as a function of two variables, psi (the product of catchment area and channel slope) and A, (the temporally averaged gully area for the peri od). Sediment input from gullies was determined from a well defined relatio nship between sediment yield and gully area. Sediment scoured from channels was estimated from dated terrace remnants and the current channel bed. Ter race remnants represent aggradation during major floods. This technique pro vides estimates of SDR averaged over periods between large magnitude terrac e-forming events and with the present channel bed. The technique averages o ut short-term variability in sediment flux. Comparison of gully area and sediment transport between two periods (1960-1 988 and 1988-1996) indicates that the annual rate of sediment yield from gu llies for the later period has decreased by 77 per cent, sediment scouring in channels has increased by 124 per cent, and sediment delivered from catc hments has decreased by 78 per cent. However, average SDR for the tributari es was found to be not significantly different between these periods. This may reflect the small number of catchments examined. It is also due to the fact that the volume of sediment scoured from channels was very small relat ive to that produced by gullies. According to the equation for SDR determined for the Waipaoa headwaters, SD R increases with increasing catchment area in the case where A, and channel slope are fixed. This is because the amount of sediment produced from a ch annel by scouring increases with increasing catchment area. However, this r elationship does not hold for the main stem of the study catchments, becaus e sediment delivered from its tributaries still continues to accumulate in the channel. Higher order channels are, in effect, at a different stage in the aggradation/degradation cycle and it will take some time until a main c hannel reflects the effects of reforestation and its bed adjusts to net deg radation. Results demonstrate significant differences among even low order catchments , and such differences will need to be taken into consideration when using SDR to estimate sediment yields. Copyright (C) 2001 John Wiley & Sons, Ltd.